1. Field of the Invention
The present disclosure relates to a control device of a lock-up clutch.
2. Description of the Related Art
Generally, a vehicle including a torque converter that transmits output torque of an engine to a transmission and a lock-up clutch that can directly connect the engine side of the torque converter to the transmission side has been known. In such a vehicle, a control device executes drive control of the state of the lock-up clutch to any of an engaged state, a released state, and a slipped state depending on an accelerator operation amount and a vehicle speed. As the slipped state of the lock-up clutch, there are a deceleration flex control state in which the differential rotation speed (input-shaft rotation speed−output-shaft rotation speed) of the lock-up clutch is controlled to a negative value when the accelerator operation amount is zero, and an acceleration flex control state in which the differential rotation speed of the lock-up clutch is controlled to a positive value when the accelerator operation amount is greater than zero.
In the conventional drive control of the lock-up clutch, when the state of the lock-up clutch is shifted from the acceleration flex control state to the deceleration flex control state in accordance with the accelerator operation amount becoming zero, a target torque capacity (a target hydraulic pressure) of the lock-up clutch has been immediately switched from the target torque capacity in the acceleration flex control state to the target torque capacity in the deceleration flex control state. However, the output torque of the engine input to the lock-up clutch decreases with a delay from a point in time when the accelerator operation amount becomes zero. Therefore, if the target torque capacity of the lock-up clutch is switched to the target torque capacity in the deceleration flex control state at a point in time when the accelerator operation amount becomes zero, the torque capacity of the lock-up clutch becomes insufficient with respect to the output torque of the engine, and the blow-up of the engine speed may occur.
Due to such background, Japanese Laid-open Patent Publication No. 2010-216506 has proposed an invention that suppresses the blow-up of the engine speed when the state of the lock-up clutch is shifted from the acceleration flex control state to the deceleration flex control state. More specifically, in the invention described in Japanese Laid-open Patent Publication No. 2010-216506, when the state of the lock-up clutch is shifted from the acceleration flex control state to the deceleration flex control state, the state of the lock-up clutch is fixed to the acceleration flex control state during a predetermined time until the output torque of the engine decreases.